1. Field of the Invention
The present invention relates to a digital multiplexer for time-division multiplexing digital signals from framing units.
2. Description of the Related Art
A digital multiplexer is a device for converting lower-order group input signals, which are asynchronous digital signals, on a plurality of channels to a higher-order group signal by time-division multiplexing. A conventional digital multiplexer is described in "Multiplexing and Synchronization Techniques," ch. 3 in a book entitled "Digital Communication Techniques," Tokai University publishing society in Japan. According to this digital multiplexer, a plurality of framing units are provided for a plurality of channels, respectively, and output signals of the framing units are time-division multiplexed by a multiplexer. In this case, the multiplexer provides to the framing units block synchronization signals each of which is delayed in time by a predetermined number of bits which differs from channel to channel. Each of the framing units produces a block signal composed of a m-bit information signal and a 1-bit block delimiter signal in synchronism with a corresponding block synchronization signal. The multiplexer multiplexes block signals sequentially sent from the framing units and adds C bits to a resultant multiplexed signal. This results in a BSI-code higher-order group signal composed of mBlC.
According to the multiplexing scheme as described above, to produce the block synchronization signals which are subjected to delays suitable for the respective channels, the multiplexer needs delay circuits corresponding in number to the channels. This will result in increased scale and cost of the multiplexer circuitry.
In addition, to design a digital multiplexer with a different number of multiplexing (the number of channels for lower-order group signals), delay circuits necessary for producing block synchronization signals must be provided in the number of signals to be multiplexed. This will render flexible alteration of the number of signals to be multiplexed difficult.